As an exhaust gas purifying system for reducing the amount of PM discharged from an engine to the outside by collecting the PM using a DPF, there is known a continuous regeneration-type DPF device constituted by a DPF and a DOC (Diesel Oxidation Catalyst) provided on the upstream side of the DPF (see Patent Document 1, for example).
In this continuous regeneration-type DPF device, when the temperature of the exhaust gas is approximately 350° C. or higher, the PM collected by the DPF is continuously burned and decreased, allowing the DPF to self-regenerate. When, on the other hand, the temperature of the exhaust gas is low, the temperature of the DOC drops, and, for this reason, the DOC cannot be activated. It is, therefore, difficult to oxidize the PM to allow the DPF to self-regenerate. As a result, the PM deposits on the DPF, clogging the DPF and increasing the pressure of the exhaust gas.
The deposition amount of PM is proportional to an output value of a differential pressure sensor for measuring the difference in exhaust gas pressure between the front side and rear side of the DPF. Thus, when the output value of the differential pressure sensor exceeds a predetermined differential pressure, an ECU (Engine Control Unit) detects clogging of the filter, thereby starting DPF regeneration.
The exhaust gas purifying system forcibly increases the temperature of the exhaust gas flowing into the DPF, by performing multiple injection (multistage delay injection) or post injection (subsequent injection) within a cylinder (tube). In this manner, the DPF regeneration is performed to burn and eliminate the PM collected by the DPF. The multiple injection is performed for the purpose of increasing the temperature of the exhaust gas discharged from the engine and increasing the temperature of the DOC up to a catalyst activation temperature. The post injection is performed for the purpose of increasing the temperature of the exhaust gas at an inlet of the DPF up to a temperature equal to or greater than a temperature at which the PM is burned, by supplying a large amount of unburned fuel into the exhaust gas and oxidizing (burning) the supplied unburned fuel by means of the DOC.
Once the DPF regeneration is started, the ECU controls fuel injection, as well as an exhaust throttle and exhaust brake valve, to increase the temperature of the exhaust gas. As a result, the PM that has deposited on the DPF is burned. In this DPF regeneration, a small amount of fuel is mixed into engine oil as a result of the post injection, generating a phenomenon called dilution. Development in dilution of the engine oil might cause engine trouble.
There is proposed another system for injecting fuel into an exhaust pipe, which injects fuel directly into the exhaust pipe through a fuel injection valve provided in the exhaust pipe, for the purpose of preventing the occurrence of dilution caused by the post injection and improving regeneration control efficiency. This system for injecting fuel into an exhaust pipe is favorable in terms of assembling an engine because an exhaust adaptor of a turbocharger of the engine is provided with a fuel injection nozzle.